The present invention relates to a method for decoding Variable Length Codes.
Usual image resp. video compression standards contain spatial resp. spatial and temporal compression. Temporal compression consists in that only the first image or one image at predefined time intervals is entirely encoded, for subsequent images only the difference to the entirely encoded image is encoded. Spatial compression usually consists in that the image is firstly applied a transform compression technology such as discrete cosine transform or wavelets and secondly applied an entropy compression technology such as Huffmann code, arithmetic code RVLC or U-VLC all belonging to the family of Variable Length Codes. The step of entropy compression will be the framework for the present invention.
A Variable Length Code comprises a plurality of codewords which are transmitted on a transmission channel to a receiver. At receiver side, the codeword dictionary is known, and the decoder separate the codewords out of the bit stream to recover originally transmitted data. A drawback of this usual decoding method is that transmission errors can propagate spatially until the decoder detects that it cannot find any codeword matching the received sequence and until the next synchronization sequence is found.
Actually, Variable Length Codes decoding method require a reliable transmission channel to be efficient. In mobile communication networks, bit errors due to non-reliable transmission medium can result in a loss of synchronization when decoding codewords. Moreover, due to real time constraints, it is not possible to protect the transmitted data with an error correction mechanism (e.g. radio link protocol) which triggers the repetition of erroneous data frames.
Known in the art are decoding methods for Variable Length Codes based on the projection of the received sequence on the codeword dictionary. Such methods are described in following articles:                On Variable Length Codes for Iterative Source-Channel Decoding, R. Bauer, J. Hagenauer, Proceedings of IEEE Data Compression Conference, 2001, page(s): 273-282.        Iterative Source-Channel Decoding based on a Trellis representation for Variable Length Codes, R. Bauer, J. Hagenauer, ISIT 2000, June 25-30, Sorrento, Italy.        
These methods exploit the relationship between bits inside the codeword. However, the relationship is not strong enough to recover efficiently errors at the receiver. Moreover, the decoded sequences may lead to not meaningful codeword sequences even if the decoding of each codeword taken individually seems correct.
A particular object of the present invention is to provide an improved method for decoding Variable Length Codes especially in communication networks having a non-reliable transmission medium.
Another object of the invention is to provide a receiver for performing this method.